1. Field of the Invention
The present invention relates to a diaphragm stopper construction for a high-pressure accumulator which defines the limit of deformation of a flexible disk-shaped metal diaphragm disposed in a high-pressure vessel which supports and seals the perimeter portion of the diaphragm to form a high-pressure chamber.
2. Description of the Related Art
Diesel engines are the most widely known of the so-called "cylinder-injected" or "direct injection engines", engines in which fuel is injected into the engine cylinder, but in recent years cylinder-injected spark ignition engines (gasoline engines) have also been proposed. Cylinder-injected engines of this kind demand that fuel pressure surges be minimized to maintain sufficiently high fuel injection pressure and ensure stable injection. To this end, compact single-cylinder high-pressure fuel pumps have been proposed which are of simple construction and inexpensive to manufacture.
However, because there is only one plunger in the single-cylinder system, there are surges of quite some amplitude in the pressure of the fuel discharged, and so surge absorption devices with metal bellows or diaphragms have been proposed to absorb these surges.
FIG. 4 shows a high-pressure fuel supply system provided with a high-pressure accumulator which is a useful example of a surge absorption device to which the diaphragm stopper construction of the present invention can be applied. In FIG. 4, a delivery pipe 1, which is a fuel injection apparatus, is provided with a plurality of injectors 1a corresponding to the number of engine cylinders, which are not shown. A high-pressure fuel pump assembly 200 provided with a high-pressure fuel pump 3 is disposed between the delivery pipe 1 and a fuel tank 2. The delivery pipe 1 and the high-pressure fuel pump 3 are connected by a high-pressure fuel passage 4 and the high-pressure fuel pump 3 and the fuel tank 2 are connected by a low-pressure fuel passage 5. Together, the high-pressure fuel passage 4 and the low-pressure fuel passage 5 compose a fuel passage connecting the delivery pipe 1 to the fuel tank 2. A filter 6 is disposed in the fuel intake of the high-pressure fuel pump 3. A check valve 7 is disposed on the fuel discharge side of the high-pressure fuel pump 3. A drain 8 attached to the high-pressure fuel pump 3 returns to the fuel tank 2.
A low-pressure fuel pump 10 is disposed at the end of the low-pressure fuel passage 5 closest to the fuel tank 2. A filter 11 is disposed in the fuel intake of the low-pressure fuel pump 10. A check valve 12 is disposed in the low-pressure fuel passage 5 on the fuel discharge side of the low-pressure fuel pump 10. A low-pressure regulator 14 is disposed in the low-pressure fuel passage 5 between the high-pressure fuel pump 3 and the low-pressure fuel pump 10. A filter 15 is disposed in the fuel intake of the low-pressure regulator 14. A drain 16 attached to the low-pressure regulator 14 returns to the fuel tank 2.
The high-pressure fuel pump 3 increases the pressure of the fuel supplied to it by the low-pressure fuel passage 5 and discharges it to the delivery pipe 1. A dumper 30 is disposed on the low-pressure fuel passage 5 side of the high-pressure fuel pump 3, i.e., the low-pressure side. A high-pressure accumulator 70 and a high-pressure regulator 32 are disposed on the high-pressure side of the high-pressure fuel pump 3. A drain 33 attached to the high-pressure regulator 32 returns to the fuel input side of the high-pressure fuel pump 3.
FIG. 5 is a cross-section showing details of the high-pressure fuel pump assembly 200 when fully assembled, comprising the high-pressure fuel pump 3, dumper 30, high-pressure accumulator 70, high-pressure regulator 32, filter 6, and check valve 7. In FIG. 5, a recess portion 40c is formed in the casing 40 on the right-hand side of the diagram, and the high-pressure accumulator 70 is secured to the recess portion 40c. A discharge passage 4b which communicates with a discharge passage 4a is formed as a recess in the bottom of the recess portion 40c.
FIG. 6 is a cross-section showing details of the high-pressure accumulator 70, which is a surge absorption device to which the diaphragm stopper construction of the present invention can be applied. The high-pressure accumulator 70 is provided with a case 85, which is a high-pressure vessel roughly the shape of a thick disk, a flexible disk-shaped metal diaphragm 86, supported by and sealed against the case 85 around its perimeter portion so that together they form a high-pressure chamber 71, and a disk-shaped plate 89, which is a stopper defining the limit of deformation of the diaphragm 86.
The case 85 has a comparatively thin perimeter portion 72, which supports and seals the outer perimeter portion of the diaphragm 86 by a sealing weld, and a comparatively thick central portion 73, in which the high-pressure chamber 71 is formed. A male thread 91 is formed on the cylindrical outer surface of the perimeter portion 72, and a comparatively shallow saucer-shaped recess portion 74, which gradually deepens from the perimeter portion towards the central portion in a smooth curve to allow the diaphragm 86 to deform towards the high-pressure chamber 71, is formed in the portion in close contact with the diaphragm 86. An approximately-cylindrical recess portion 75, which communicates with the shallow saucer-shaped recess portion 74 at the central portion, is formed in the central portion 73 and, together with the saucer-shaped recess portion 74, forms the high-pressure chamber 71.
A gas charge inlet 84 of circular cross-section about its central axis is formed in the ceiling portion of the high-pressure chamber 71 to introduce high-pressure gas to the high-pressure chamber 71 of the case 85 and seal it in, and a sealing device 87 is disposed therein to seal the gas charge inlet 84. The gas charge inlet 84 is provided with a small-diameter portion 76 of comparatively small diameter on the high-pressure side facing the high-pressure chamber 71, and a large-diameter portion 77 of comparatively large diameter on the low-pressure side facing the exterior of the case 85. A shoulder portion 78 is formed between the small-diameter portion 76 and the large-diameter portion 77, and a female thread is formed on the inner surface of the small-diameter portion 76. An annular groove 79 is disposed in the shoulder portion 78 to accommodate an O-ring 88.
The sealing device 87 is a plug member inserted into the described gas charge inlet 84 and has a large-diameter portion 81, which is inserted into the large-diameter portion 77 of the gas charge inlet 84, and a small-diameter portion 80, which has a thread around its outside surface which engages the female thread of the small-diameter portion 76, and the large-diameter portion 81 inserted into the gas charge inlet 84 presses on the O-ring 88 and seals the gas charge inlet 84.
The perimeter portion of the diaphragm 86 is sealed and supported on the outer perimeter portion of the case 85 by a weld portion 82 made by an electron beam or the like, but in addition a saucer-shaped plate 89 is disposed on the diaphragm 86 as a stopper to define the limit of deformation of the diaphragm 86, and the plate 89 is also fastened around its circumference by the weld portion 82. A recess portion 83 shaped like one side of a convex lens is formed on the inner face of the plate 89, which gradually deepens from the outer perimeter portion of the diaphragm 86 towards the center, and communicating holes 90 are formed as fuel channels which communicate with the recess portion 83.
The case 85, the metal diaphragm 86, and the plate 89 are all hermetically sealed and bonded to each other around their outer perimeter portions by welding with an electron beam, or the like. The space sealed between the metal diaphragm 86 and the case 85 is charged with a high-pressure gas such as nitrogen.
In the high-pressure fuel pump assembly 200 in FIG. 5, a male thread 91 formed around the outside of the case 85 engages a corresponding female thread formed in the recess portion 40c, and the high-pressure accumulator 70 is inserted into the plate 89, sealed by an O-ring 51, and secured to the recess portion 40c so as to allow the communicating holes 90 to communicate with the discharge passage 4b.
The high-pressure accumulator 70 constructed in this way absorbs surges in the pressure of the fuel discharged by the discharge passage 4b. That is, while fuel is being discharged through the discharge passage 4b, surges occur in the discharge passage 4b, for example, when the high-pressure fuel pump is operating. The volume of the high-pressure chamber 71 varies in response to changes caused by the surges until the pressure of the high-pressure gas in the high-pressure chamber 71 reaches equilibrium with the pressure in the discharge passage 4b through the diaphragm 86. For example, when the pressure in the discharge passage 4b rises, the diaphragm 86 is deformed such that the volume of the high-pressure chamber 71 decreases and the volume of the discharge passage 4b increases, and so the pressure in the discharge passage 4b decreases and surging is reduced.
When an engine stops, the supply of fuel from the high-pressure fuel pump 3 also stops, and the fuel pressure in the lens-shaped recess 83 on the plate 89 side gently decreases. For that reason, the diaphragm 86 is displaced from its position during normal operation shown in the diagram due to the pressure of the gas in the high-pressure chamber 71, but to prevent damage and wear on the diaphragm 86, a diaphragm stopper construction is employed having a curve such that when the diaphragm deforms a certain amount, it comes into contact with the surface of curve of the lens-shaped recess 83 on the plate 89 and does not deform any further, and thus excessive stress does not concentrate on the diaphragm 86.
In a conventional high-pressure accumulator, the plate which the diaphragm comes into contact with is a diaphragm stopper construction which defines the limit of deformation of the diaphragm in order to prevent damage to the diaphragm caused by a large displacement of the diaphragm due to gas pressure in the high-pressure chamber when the engine stops, and its shape has previously been determined on the basis of the deflection of the diaphragm calculated using equations for the deflection of a disk which are well known in material mechanics. The shape of the contact surface used to be defined, for example, based on the equation expressing deflection when a disk secured around its circumference is subjected to a uniformly distributed load or the equation expressing large deflection when a disk secured around its circumference is subjected to a uniformly distributed load, as described in the JSME Handbook for Mechanical Engineer's: Material Mechanics, Sixth Edition (compiled by the Japan Society of Mechanical Engineers).
However, it has been discovered that if the shape used for the contact surface is based on deflection derived from the equation expressing deflection when a disk secured around its circumference is subjected to a uniformly distributed load, stress in the central portion of the diaphragm is high, and even though there may be a strong margin for stress in other portions, the diaphragm may be damaged or ruptured starting at the central portion where stress is locally intense. On the other hand, it has been discovered that if the shape used for the contact surface is based on deflection derived from the equation expressing large deflection when a disk secured around its circumference is subjected to a uniformly distributed load, stress around the perimeter portion of the diaphragm is high, and even though there may be a strong margin for stress in other portions, the diaphragm may be damaged or ruptured starting at the perimeter portion where stress is locally intense.
Also, when used in an environment with a large range of working temperatures, the pressure of the high-pressure gas in the high-pressure chamber 71 changes with changes in working temperature and the operating position of the diaphragm 86 changes. The range of possible changes in volume of the diaphragm 86 is determined by the saucer-shaped recess 74 and the lens-shaped recess 83, so that when the change in the pressure of the high-pressure gas in the high-pressure chamber 71 is great there is a possibility that the diaphragm 86 may come into contact with either the saucer-shaped recess 74 or the lens-shaped recess 83 and fail to function as an accumulator. In order to increase the working range of the high-pressure accumulator, it is necessary to increase the volume of the saucer-shaped recess 74 and the lens-shaped recess 83, and this has conventionally been done by increasing the diameter. In that case, however, the external dimensions of the high-pressure accumulator become too large.